1. Field of the Invention
The present invention relates to a process of producing lightweight and corrosion-resistant heat exchangers suited for use in, for example, air conditioners for vehicles such as automobiles.
2. Description of the Prior Art
In the automobile industry, it is quite an important requirement to reduce the weights of automotive component parts for the purpose of improving fuel economy and, therefore, there is an increasing demand for lightweight heat exchangers, e.g., condensers and evaporators, for automotive air conditioners.
A typical conventional process for producing such a heat exchanger has the following steps:
(i) forming a continuous flat tube having a plurality of refrigerant passage bores by extrusion from a lightweight metal such as aluminum or its alloy, and bending or winding the flat tube to form a plurality of parallel runs;
(ii) placing corrugated fin members of an aluminum alloy each between an adjacent pair of the runs of the flat tube to form an assembly, the fin members each having an extremely small thickness on the order of 0.16 to 0.18 mm and being coated beforehand on their surfaces with a brazing material; and
(iii) placing the assembly of the flat tube and the corrugated fin members in a heating oven with the aid of a suitable jig and heating the assembly to a temperature higher than the melting temperature of the brazing material to secure, by brazing, the corrugated fin members to the flat tube.
The aluminum alloy used as the material of the corrugated fin members is a "base" material having a lower electrode potential value than the material of the tube so that the corrugated fin members are corroded earlier than the tube when the heat exchanger is used under a corrosive condition, thus providing an effect which is generally known as "sacrificial corrosion effect". Namely, while corrosion of the fin members does not cauase any serious problem, even a small pin hole formed in the tube wall due to corrosion makes the heat exchanger as a whole unusable.
On the other hand, the brazing material applied beforehand to the surface of the fin member contains a large quantity of silicon which is added for the purpose of reducing the melting point of the brazing material. During the brazing, the silicon is undesirably diffused into the fin material to lower the melting point of the fin member itself. As a result, the fin member tends to be buckled by the pressure which is applied by the jig during the brazing. This is the reason why the thickness of the corrugated fin member could not be reduced.
Referring now to the tube, the aforementioned sacrificial corrosion effect which is provided by the corrugated fin members cannot be expected in the bent portions of the tube because the fin members are not present in such portions. In order to prevent corrosion of the bent portions of the tube, therefore, various measures have been proposed which include attaching sacrificial plates or applying a sacrificial coating material to the bent portions of the tube. These measures, however, are not desirable because the production cost is raised significantly.
Another measure for preventing the corrosion of the tube is to subject the tube surface to a zinc diffusion treatment. This method, however, encounters a problem due to difficulty in the disposal of the effluent from the treating bath.
In order to obviate these problems, the present inventors have alreadly attempted a process for producing a heat exchanger in which the brazing material has been applied not to the thin corrugated fin members but to the tube itself which has a wall thickness much greater than that of the corrugated fin member, thus forming on the tube a clad of the brazing material and avoiding the aforementioned difficulty caused by the reduction in the melting point attributable to the diffusion of silicon. This process, however, has suffered from the following disadvantages. Namely, in contrast to the case where the brazing material is simply applied to a planar sheet material forming the corrugated fin members, the cladding of the tube with the brazing material caused a problem that, during the working of a sheet metal into an extruded flat tube with a plurality of passage bores, a part of the brazing material is moved into the passage bores. It is often experienced that the effective cross-sectional area of the passage bores is reduced due to re-melting of the brazing material by the heat which is applied during the brazing. In addition, after the brazing, a time-consuming and troublesome rinsing work is required to remove the flux material which is usually of the conventional, highly corrosive one containing ZnCl.sub.2 as a main component.